this article for Nolo, and a competing legal site later plagiarized it. Copyright infringement is when someone makes unauthorized use of your copyrighted material. Attribution has zero effect on determining whether copyright infringement has occurred -- it's kind of like if someone stole your car but was nice enough to paint your name on the side; you're still the victim of car theft. It's possible that attribution may be considered as a mitigating factor when determining damages for copyright infringement (which is more likely if the borrower sought to locate the copyright owner but failed) or a judge may consider it an aggravating factor (proof that the borrower knew you were the author but went ahead and ripped you off anyway).
How do you deal with theft of a blog entry? That's a tough call. You can write to the borrower and ask them to remove it, citing your copyright ownership. You can exert rights under the DMCA and have the borrower's ISP take down the entry. But filing a lawsuit would probably not be worth your efforts unless you could demonstrate a financial injury that would justify the attorney fees.